1. Field of the Invention
This invention relates generally to the processing of electronic article surveillance (EAS) tag signals, and more particularly to a system and method for reducing circuit ringdown time for a switching amplifier used within an EAS transmitter signal generator.
2. Description of the Related Art
An acoustic-magnetic or magneto-mechanical EAS system excites an EAS tag by transmitting an electromagnetic burst at a resonance frequency of the tag. The tag responds with an acoustic-magnetic or magneto-mechanical response frequency that is detectable by the EAS system receiver. At the end of the transmitter burst, the system detects the exponentially decaying response of the tag. However, because the tag signal amplitude rapidly decays to ambient noise levels, the time interval in which the tag signal can be detected is limited.
In such systems, the transmitter burst signal does not end abruptly, but instead decays exponentially because of transmitter circuit reactance. As a result, it is difficult to detect the tag signal until this circuit “ringdown” has essentially disappeared. Therefore, the time period during which the tag signal can be detected is reduced. This is a particular problem because the circuit ringdown occurs while the tag signal is at its largest.
U.S. Pat. No. 4,510,489 discloses such an EAS system, one embodiment of which is sold under the trademark ULTRAMAX by Sensormatic Electronics Corporation, Boca Raton, Fla. The ULTRAMAX system uses a pulsed transceiver operating at a particular frequency with a nominal pulse duration. Following the pulse, a receiver portion “listens” for the presence of a tag signal. The load that the power amplifier sees is a high-Q resonant circuit. At the end of the transmit burst, the transmitter signal follows the natural response of the antenna, which is a slow decay of the transmit power. The transmitter signal decays slowly because transmission of a signal results in an electromagnetic field surrounding the transmission antenna. After transmission is completed, the electromagnetic field begins to collapse, the result of this collapsing field is currents being induced within the transmitter.
However, this decay of the transmit signal sometimes interferes with tag reception, because the tag also operates at a frequency approximate that of the transmit signal. The tag signal and the decaying transmitter signal may also overlap in both time and frequency, so it is very difficult to separate the two signals. Furthermore, left to its natural response, the period it takes for the decaying transmit signal to become smaller than the tag signal may cause operational difficulties for the EAS system.
Previous solutions for the circuit ringdown problem have been to switch the transmitter portion of the transceiver into a “de-Q'ing” circuit at the end of the transmit burst time (e.g., at 1.6 ms) in order to reduce the “Q”, or quality factor, of the antenna load, for example, from about 25 to about 2. The transmit signal then decays much faster, allowing for earlier detection of the tag signal. However, stored energy in the transmit antenna (the collapsing electromagnetic field) is dissipated in the de-Qing circuit. This stored energy can result in a substantial amount of power to be dissipated and the physical size and cost of the components in the de-Qing circuit can become quite large.